In Situ Nanoscale Probing of Lithium‐Aluminum Alloying / De‐Alloying Kinetics and Mechanical Failure in All‐Solid‐State Batteries

Abstract Alloy anodes with high specific capacity are extensively utilized in all‐solid‐state batteries (ASSBs). However, they are challenged by interfacial kinetic and mechanical issues. Real‐time investigation of interfacial failure mechanisms at the nanoscale is crucial for optimizing the alloy anodes. Utilizing the high spatial resolution and real‐time imaging capabilities of electrochemical atomic force microscopy (EC‐AFM), we discovered that Li1Al1 alloying unevenly, and the delithiated phase Al with its sluggish kinetics hinders the de‐alloying processes. Combining the high mechanical modulus of Li1Al1 and Al leads to electrode fracture. This kinetic‐mechanical coupling failure diminishes the reversibility of the Al anode. To weaken the kinetic‐mechanical coupling failure, we employ a co‐sintering reaction between Al and Li6PS5Cl (LPSCl), introducing Al2S3 and P2S74−, followed by Al2S3 in situ lithiation to Li9Al4. This process improved interfacial charge transfer and mitigated mechanical failure. Consequently, the Li‐anode‐less ASSBs maintain 90.2% retention rate after 2000 h (420 cycles) and 87.4% retention rate after 3500 h (750 cycles) at an areal capacity of 2.9 mAh cm−2 and low N/P ratio of 1.8 with a high average coulombic efficiency of 99.98%. Such tracking of the alloy interfacial reaction provides an in‐depth understanding of kinetic‐mechanical coupled failure and thus benefits the alloy anode optimization.